1. Field of the Invention
The present invention relates generally to implantable prostheses for replacing human skeletal joints, and relates more particularly to an anti-backout screw for use with an implantable prosthesis.
2. Background Art
Implantable orthopedic prostheses, in one form, comprise manufactured replacements for the ends and articulating surfaces of the bones of the skeleton. Such prostheses are implanted to repair or reconstruct all or part of an articulating skeletal joint that is functioning abnormally due to disease, trauma, or congenital defect. Among the various articulating skeletal joints of the human body that are eligible to be fitted with implantable orthopedic prostheses, the knee joint is often treated with such prostheses. The knee joint is a major weight bearing joint and degenerates more quickly than some other joints in case of abnormality. Also, the knee joint plays a critical role in ambulation and quality of life, resulting in great demand for surgical correction of abnormalities.
The human knee joint involves three bones: the femur, the tibia and the patella, each having smooth articulation surfaces arranged for articulation on an adjacent articulation surface of at least one other bone. The femur includes at its distal extremity an articulation surface having medial and lateral convex condyles separated posteriorly by an intercondylar groove running generally in the anterior-posterior direction. The condyles join at the distal-anterior face of the femur to form a patellar surface having a shallow vertical groove as an extension of the intercondylar groove. The patella includes on its posterior face an articulation surface having a vertical ridge separating medial and lateral convex facets, which facets articulate against the patellar surface of the femur and against the medial and lateral condyles during flexion of the knee joint, while the vertical ridge rides within the intercondylar groove to prevent lateral displacement of the patella during flexion. The tibia includes at its proximal end an articulation surface having medial and lateral meniscal condyles that articulate against the medial and lateral condyles, respectively, of the femur. The mutually engaging articulation surfaces of the femur and the patella together form, functionally, the patellofemoral joint, and the mutually engaging articulation surfaces of the femur and tibia together form, functionally, the tibiofemoral joint, which two functional joints together form the anatomical knee joint.
One or more of the articulation surfaces of the knee joint may fail to act properly, requiring the defective natural articulation surface to be replaced with a prosthetic articulation surface provided by an implantable prosthesis. To fit defects of varying scope, while allowing healthy portions of the knee joint to be conserved, a range of types of orthopedic knee implants is available. The range extends from total knee prosthesis systems for replacing the entire articulation surface of each of the femur, tibia and patella, to simpler systems for replacing only the tibiofemoral joint, or only one side (medial or lateral) of the tibiofemoral joint, or only the patellofemoral joint. Commonly employed orthopedic knee prostheses include components that fall within one of three principle categories: femoral components, tibial components, and patellar components. A so-called "total" knee prosthesis includes components from each of these categories. The femoral component replaces the distal end and condylar articulating surfaces of the femur and may include a proximal stem received within the medullary canal at the distal end of the femur. The tibial component replaces the proximal end and meniscal articulating surfaces of the tibia and may include a distal stem received within the medullary canal at the proximal end of the tibia. In some designs, the proximal stem of the femoral component or the distal stem of the tibial component is optional and is provided as a modular component. The connection between the femoral or tibial component and the respective proximal or distal stem is often accomplished via friction locking male and female conical tapered surfaces on the respective elements, also known as a Morse taper connection. Sometimes, the locking taper connection is further secured by a threaded screw extending through one element and threadedly received in the other element. The patellar component replaces the posterior side and natural articulating surface of the patella. Sometimes, the patellar component is not used, and the natural articulating surface of the patella is allowed to articulate against the femoral component.
Where a locking taper connection is employed in an implantable prosthesis, such as between a femoral or tibial component and a respective medullary stem, and further secured by a retaining screw, it is desirable that the screw be secured against backing out of the threaded bore in which it resides. Under the strain of load, the retaining screw could become loose in its threads and begin to back out, or unscrew. If the screw were to back out too far, the head of the screw could interfere with the articulating surfaces of the prosthetic joint. To avoid such an occurrence, a locking means is often employed to prevent the screw from backing out. Typically, the locking means comprises a fourth element, inserted after the screw is tightened, to cover the head of the screw and restrain the screw in its appointed location.
Using a fourth element to retain the screw has certain disadvantages: an additional step is required to insert the fourth element, thereby prolonging the implantation surgery; and the fourth element is usually small and prone to being dropped or lost during surgery. It would be desirable to provide a retaining screw, with anti-backout characteristics, that overcomes these disadvantages.